Measurement of resolving power of schmidt telescopes



NOW 1947. L. T, SACHTLEBEN MEASUREMENT OF RESOLVING POWER OF SCHMIDT TELESCOPES Filed Dec. 28, 1943 2 Sheets-Sheet 1 INVENTOR.

lazzirenmffiwktbm Nov. 25, 1947. L. T. SACHTLEBEN MEASUREMENT OF RESOLVING POWER OF SCHMIDT TELESCOPES Filed Dec. 28, 1945 I 2 Sheets-Sheet 2 0 0% O .0 o i oQo N 050 o 0. o o o o o NV N w W 0 O o O o 0 00 o a 0 o 0W0 N o 0 O 0 n O 0 0W0 w cho 0W0 ON W0 o o o a o o o o v N Q N 0& w 0 O o o 0 o 0 O O 0 O gomo 0W0 o 0W0 emu o 0%: M a o 0 0 on e Qo o IN VEN TOR. Zazvrcml'fiacllllefim Patented Nov. 25, 1947 U I E S A E PA E T Q' EE MEASUREMENT OF RESOLVING POWER OF SQHMIDT TEI ESCOPES Lawrence T. Sachtleben, Indianapolis, Ind., assignor to Radio Corporation of America, a corporationof Delaware Application December 28, 1943, Serial No.

3 Claims.

1 This invention involves a method of testing Schmidt telescopes or other'optical devices using an annular objective. There are various methodjsfof testing optical instruments which are (01. ss-ss 2 a parallel line pattern is illustrated for example in Some televisionproblems from the motion picture standpoint, (GIL. Beers, W, Engstrorn, andJ. Malofi) J. s. M. n, XXXII adapted for use with circular objectives but 5 (Feb. 1939.), p. 121. Line, patterns do not become which are totally unsuited for testing optical wholly unresolved when an opticalsystein i instruments with annular objectives such as the ing them goes out of focus" but for certain out-of- Schmidt telescope, for example. Some of these focus conditions the line pattern reappears, methods of testing may be used in the laborathough not as welldefined as when in S ll tory and with appropriate laboratory precautions focus. When the optical system has an annular accompanying their use, but are totally unsuited pupil as in the Schmidt telescope the angle of for production tests of such apparatuses with convergence of the extreme opposite edge rays comparatively unskilled personnel. This is -parmay be large while for'asmall sector of the annuticularly true with methods utilizing test objects ms" the angle of convergence oi the edge rays consisting of substantially parallel lines, which is may be rather small. For example, in one type the most usual form in use. of Schmidt' telescope for ultimate visual observa- In the present invention a chart is made of tion the ebn-vrg e'nceer the'extreme ed e rays such size that when viewed through a collimator from opposite sides of the annular aperture is it fills the field of view of a Schmidt telescope. about 86, while the extreme edge rays from a This chart is divided into small square units each small sector of the annulus converge at a angl of which contains detail in a range of sizes, the of about 18. The depth of focus of the whole detail consisting of an'annular'figiure whose outape'rture is therefore far less than that Q side diameter is three times the inside. diameter. limited sector 'o fthe apertu e- A figure is considered to be resolved when it is Referring now to Fig. 1, if P represents l e to a e a c ter and d s n t appear as annular 'e'gi pupil bran o tical s st m ha ng simply a solid dot which displays no structure an annular objective in the XY p ane, W l 8 within itself. The resolving power of the telepattern of parallel lines T, U, V, and W is imaged S QODE will be two-thirds of the angular diameter on the optical axis Z; at L1; as t, v, and w in oi the smallest annulus resolved in any given gplane parallel to by the optical system, part of the field. By angular diameter is meant 0 it will be apparerrt that the segments oi the the angle subtended at the pupil of the optical annulus cut off by the lines AC and B1) constiinstrument by the external diameter of the retute the major portion of the area of the pupil s v u s. a he mien s n. firstf n nm tiqn eq iva- One. object of the invention is to provide an to two lo g 5 9th Per @1 3 A f d B p ved method for testing optical instruments positions corresponding to those of the segments with annular objectives, defined above. The depth of focus of either Another b t Of the invention is tov provide a slot alone for the lines parallel to its long dimenmethod of directly determining the, resolving 1.9 s g a but 9? he w eth is W 3 Power of optical instruments. 5.31%11- T P65 1 is hat if a. 3 1 eivin Another object of the invention is to provide 4 0 h pa allel n ma LL .5. mPY d ar a method of directly determining the resolving or away from the plane of the true image the power of optical instruments with annular obl n s first go out of focus and then ar b u t ieetivesnub ort a m ut not quite a sharply a Other and incidental objects of the invention before will be apparent to those skilled in the art from a Referring now to Fig; M 42d N e s n reading of the following specification and n the narrow dimensions oi: the two parallel slots inspection of the accompanying drawings in dd 9 in QQQQ QB w 1 defined which: by 0 and B True images of t lines will Figure 1 is a diagrammatic view of an optical be formed in the image planeI at t, a, v and w. instrument having an annular objective set for The lines drawn from the centers oi the slots testing with parallel lines. M and N to each of the lines T, U, etc. are p Figure 2 is a diagram of the images formed cipal raysQthere being two for each line T, of the parallel lines in an arrangement as illus- V and W in the ob ect plane. orresponding 1 trated in Fig. 1. one of the lines t, u, v and w n th t ue im Figure 3 is an illustration of one element of In the true image plane I, the two principal rays my improved test object, and from a line T through M and N intersect in the Figure 4 is an illustration of a screen composed liner. The same relation holds between each of of a plurality of such test objects. the other lines in the object plane and the cor- Parallel line patterns are frequently used to responding line in the image plane. If we shift test the resolving power of optical systems. Such 69 from the plane I to the plane 11, it is seen that the principal ray from T through M intersects the principal ray from U through N, and that from U through M intersects that from V through N,

etc. Since the lines in the object space are regu-- larly spaced the images reappear at 11 and when the focal depths of M and N are great the false image at I1 may be almost as good as that at I. In a plane midway between I and I1, the lines will be definitely out of focus and not resolved. False images also occur at 12, I3 and I; for the same reasons.

The image of a Schmidt telescope lies on a spherical surface and must therefore be received on a spherical photographic plate, light responsive surface or diffusing screen, or must be viewed with an eyepiece especially designed for such a spherical image surface. In testing a Schmidt telescope the image is received on a spherical difiusing screen and viewed with a microscope. The quality is judged largely by the fineness of detail resolved in the image. It is readily seen from the foregoing that a flaw in the curvature of the image may occur without seriously aifecting the apparent resolution of the H finest parallel line detail in certain parts of the field. In such a case where the image is obviously faulty but the finest detail is still resolved, it is most difilcult to interpret the results and therefore a parallel line system or a test object having the same characteristics as to imis easily resolved by any except the most faulty optical system being tested, the second, 2, being of half that size, the third, 3, being of one-third that size, etc. As indicated above, an annulus is resolved when the center is apparent and the resolving power is two-thirds of the angular diameter of the smallest annulus resolved'so that with each two successive diameters of annulus being in the ratio of the reciprocals of their order numbers, the order number provides a direct measure of the resolving power of the instru- As indicated above, the diameter of the ment. center is one-third of the external diameter of the annulus. The test object of Fig. 3 is in its present form made in the form of a square about 5% on each side, and for the type of Schmidt telescope referred to above 48 of these test objects are pasted upon a screen as illustrated in Fig. 4 and a suitably reduced photographic image of this screen is illuminated by transmission. For convenience, as illustrated in Fig. 4, these test objects are numbered horizontally and lettered vertically so that the resolving power of the instrument in any specified part of the field may be represented by the letter and number of the particular test object and the number of the smallest annulus resolved.

It will be apparent that these test objects may be made much larger or smaller than the size above indicated, depending on the circumstances of their use. The largest annulus preferably should be resolved by an optical system which does not quite meet specifications at the edge of 4 the field and the smallest annulus should be smaller than the limit of the specifications at the center of the field. As many of these test objects may be used as are necessary to cover the entire field of the instrument being tested.

This pattern provides a better test of the power of an optical system to resolve heterogeneous detail than does a y type of parallel l p tt rn and does not exhibit false focus with the annular lens. It is easy to interpret and observers agree well as to indications obtained. It is also a more severe test of the optical system and does notindicate resolving powers as high as those indicated by parallel line patterns for the reason that the false images resulting from the use of parallel line test patterns, as explained previously, are eliminated.

It will be apparent that the test objects need not be opaque annuli on a translucent background but may be translucent annuli on an opaque or black background.

It will be .apparent that my test object and method of testing are not limited to use in the field of optical instruments having annular objectives but may be equally well used with practically any type of optical instrument.

I claim as my invention:

1. The method of measuring the resolving power of an optical instrument having an objective with an annular pupil, which includes 68- tablishing a test pattern comprising a plurality of annuli of graduated sizes, each of said 'annuli having an external diameter three times as great as its internal diameter, focusing an image of said pattern with'the optical instrument, determining the angular diameter of the smallest annulus resolved by the instrument, and calculating the resolving power of the instrument as two-thirds of said angular diameter.

2. The method of testing an optical instrument having an objective with an annular pupil, whereby the occurrence of false images is avoided, comprising establishing a plurality of annular test patterns of different sizes, focussing an'image of said patterns with the optical instrument, determining the angular diameter of the annular test pattern the focussed image of which is just resolved by said instrument, and calculating the resolving power of said instrument as a function of said angular diameter.

3. The method of testing an optical instrument having an annular objective with an annular pupil, whereby the occurrence of false images is avoided, comprising establishing a plurality of annular test patterns of graduated sizes, focussing an image of said patterns with the optical instrument, determining the diameter of the annular test pattern the focussed image of which is just resolved. by said instrument, and utiliz- 60 ing the relative size of said diameter as a measure of the resolving power of the instrument.

LAWRENCE T. SACHTLEBEN.

REFERENCES CITED 55 The following references areof record in the file of this patent:

UNITED STATES PATENTS Number Name Date 70 2,184,920 Bigelow Dec. 26, 1939 1,386,663 Twyman Aug. 9, 1921 

